Abstract : The behavioral effects of the anticonvulsant and neuroprotective agent MK-801 were studied under a complex task designed to mimic counting in rats. A percentile schedule arranged reinforcement for runs of consecutive left-lever responses terminated by a single right-lever response when the number of runs was closer to the target value of 12 than two-thirds of the subject's recent runs. A second group acquired differentiated runs in the same fashion, but were subsequently shifted to a procedure which yoked reinforcement probability per run to previous percentile performance. Effects of MK-801 under the two procedures were indistinguishable, even though behavioral disruption produced only a relatively minor change in reinforcement frequency under the percentile schedule while all but abolishing reinforcement under the yoked procedure. Both isomers produced dose-related decreases in run length and in response rate, with the (+) isomer being approximately one log unit more potent than the (-) isomer.